The present invention relates to a developing apparatus causing a developer carrying body arranged in an electronic copying machine to carry a developer and apply it to a charged latent image formed on a surface of an image carrying body, thereby developing the latent image.
A conventional developing apparatus of this type has a developing roller as a developer carrying body in which a plurality of magnetic poles are arranged in a rotating sleeve. A magnetic brush of a developer is formed on the rotary sleeve and is brought into slidable contact with the latent image formed on a photosensitive drum (an image carrying body) upon rotation of the rotating sleeve, thereby developing the latent image on the photosensitive drum.
The magnetic poles of the developing roller can be shifted in the radial direction with respect to the rotating sleeve. The magnetic poles are displaced in the radial direction with respect to the rotating sleeve in a development stop state so that conveyance of the developer is stopped.
In the conventional developing apparatus having the arrangement described above, conveyance of the developer is stopped by only radial displacement of the magnetic poles. For this reason, developer conveyance cannot be surely interrupted due to ambient conditions, fatigue of developer, and density of developer, thereby degrading the reliability of the developing apparatus. In particular, when developing rollers are vertically aligned in a plurality of stages, as in a color copying machine, developer which falls from an upper developing roller is attached to a lower developing roller and contaminates the latent image on the photosensitive drum. In addition, the developer is kept attached to the developing roller and is stacked thereon to disable further generation of a developing bias voltage thereon. As a result, the density of an image formed on the photosensitive drum is lowered to result in an unclear, fogged image. In addition, the magnetic characteristics of the developer used in the developing apparatus are limited to narrow the selection range for developing materials of developer.
A so-called reversal developing type developing apparatus has been recently used to perform a reversal development which is opposite to a normal development according to normal electrophotographic techniques. The reversal developing type developing apparatus develops light and dark portions in an original into dark and light portions in a printing, respectively. The conventional reversal developing type developing apparatus is used in a copying printer with the copying machine function and the printer function, a microfilm reader/printer, or a laser printer.
When a conventional electrophotographic process according to the Carlson method is used in a conventional reversal developing type developing apparatus of this type, the following problems are presented.
In reversal development, at first a photosensitive drum as an image carrying body is positively charged at a surface potential of 800 V and, then, the surface potential of a region corresponding to a bright portion of a negative original is decreased by radiation with light. A positively charged toner is attached to the exposed portion to develop the latent image. For this reason, if an original is a drawing (most of the area is a bright or light portion), the surface potential in the most surface area of the photosensitive body does not decrease in reversal development but is kept high. A developing bias voltage of the developing roller as the developer carrying body for carrying and conveying the developer is a maximum of about 500 V due to the problems of the electrostatic characteristics of the photosensitive body, the withstand voltage, and background fogging. For this reason, a strong electric field from the high-potential photosensitive body to the developing roller is formed. The positively charged toner is then attracted to the developing roller in the electric field, thereby forming a toner layer on the outer surface of the developing roller. Once the toner layer is formed, it repeatedly passes through the electric field by rotation of the developing roller and is increased in the thickness. A large local voltage drop also occurs at contact points between individual toner particles in the toner layer, between the toner and the surface of the developing roller, or between the toner layer and the toner carrier. Therefor, positive and negative charges are stored at both sides of each contact point due to dielectric polarization (i.e., a kind of capacitor is formed). A high electric field is formed at the contact point as a boundary, and a strong cohesion force acts between the toner particles due to the Johnsen-Rahbech effect caused by the strong Maxwell distorsion force.
When a solid black image (an image consisting of only a black portion) is developed by reversal development, the density of an edge portion (i.e., an image boundary) in the image is higher than that of the central portion, thus resulting in a so-called edge effect. In order to reduce the edge effect to obtain an image having a uniform density, a toner having a relatively low charging level is used.
However, when such a toner is used, the toner layer tends to be formed on the outer surface of the developing roller and affects the development performance. A typical example of such an unfavorable influence is the lowing of the developing power (that is, the lowing of the density of an image) caused by the accumulation electric charge on the toner layer formed on the developing roller when the developer comprises a one-component developer (specially, a nonmagnetic toner). On the other hand, if a two-component developer consisting of a toner and a carrier is used, a toner layer is formed on the developing roller by the toner separated from the carrier. The toner layer serves as an insulating layer and disables generation of the developing bias voltage. As a result, the image developed on the surface of the photosensitive drum has a low density, resulting in an unclear, fogged image.